Ions which have the same initial kinetic energy but different masses will separate when allowed to drift down a field free region. This is a basic principle of typical time-of-flight mass spectrometers. Ions are conventionally extracted from an ion source in small packets. The ions acquire different velocities according to the mass-to-charge ratio of the ions. Lighter ions will arrive at a detector prior to high-mass ions. Determining the time of flight of the ions across a propagation path permits the determination of the masses of different ions. The propagation path may be circular or helical, as in cyclotron resonance spectrometry, but typically linear propagation paths are used for chromatography mass spectrometry applications.
Time-of-flight mass spectrometry is used to form a mass spectrum for ions contained in a sample of interest. Conventionally, the sample is divided into packets of ions that are launched along the propagation path using a pulse-and-wait approach. In releasing packets, one concern is that the lighter and faster ions of a trailing packet will pass the heavier and slower ions of a preceding packet. Using the traditional pulse-and-wait approach, the release of an ion packet is timed to ensure that the ions of a preceding packet reach the detector before any overlap can occur. Thus, the periods between packets is relatively long. If ions are being generated continuously, only a small percentage of the ions undergo detection. A significant amount of sample material is thereby wasted. The loss in efficiency and sensitivity can be reduced by storing ions that are generated between the launching of individual packets, but the storage approach carries some disadvantages.
Resolution is an important consideration in the design and operation of a mass spectrometer for ion analysis. The tradition pulse-and-wait approach in releasing packets of ions enables resolution of ions of different masses by separating the ions into discernible groups. However, other factors are also involved in determining the resolution of a mass spectrometry system. "Space resolution" is the ability of the system to resolve ions of different masses despite an initial spatial position distribution within an ion source from which the packets are extracted. Differences in starting position will affect the time required for traversing a propagation path. "Energy resolution" is the ability of the system to resolve ions of different mass despite an initial velocity distribution. Different starting velocities will affect the time required for traversing the propagation path. Outside of the realm of ion analysis, continuous neutron beams have been modulated by mechanical choppers to increase the "on" time beyond a pulse-and-wait approach. See for example, (1) K. Skold, "A Mechanical Correlation Chopper for Thermal Neutron Spectroscopy," Nuclear Instruments and Methods, 63 (1968), pages 114-116; (2) G. Wilhelmi et al., "Binary Sequences and Error Analysis for Pseudo-Statistical Neutron Modulators with Different Duty Cycles," Nuclear Instruments and Methods, 81 (1970), pages 36-44; and (3) J. R. D. Copley, "Optimized Design of the Chopper Disks and the Neutron Guide in a Disk Chopper Neutron Time-of-Flight Spectrometer," Nuclear Instruments and Methods in Physics Research, A291 (1990), pages 519-532. The mechanical choppers release pulses of neutrons at a frequency greater than that of a pulse-and-wait approach, but the technique does not address space resolution or velocity resolution. The resolution of the system is controlled by the longest pulse used in the sequence. Moreover, it is believed that increases in the duty cycle beyond the pulse-and-wait approach are soon accompanied by a susceptibility of the system to reaching an unacceptably low level of sensitivity to low-concentration neutrons.
What is needed is a method and apparatus for analyzing ions such that increased efficiency is accompanied by increased sensitivity to low-concentration ions of a sample of interest.